A US watchdog has filed a complaint against acting Pentagon chief Patrick Shanahan over his alleged lobbying of Boeing, his former employer. The company is still reeling from the recent Ethiopia crash, that sent its stock sliding.

The complaint was filed by the Citizens for Responsibility and Ethics in Washington (CREW), a non-profit US government ethics watchdog, on Wednesday, just as the US Federal Aviation Authority (FAA) joined the rest of the world in grounding all Boeing 737 MAX 8 and 9 models over safety concerns.

The group cited numerous media reports that Shanahan, who used to work in various subdivisions at Boeing for more than 30 years before he was handpicked by President Donald Trump to serve as Deputy Secretary of Defense, has been doing the airspace manufacturer's bidding at internal meetings and in private.

After former Pentagon chief James Mattis stepped down in late 2018, Shanahan took over the Pentagon as Acting Defense Secretary. Before embarking on a career in the Trump administration, Shanahan signed an ethics agreement that precludes him from getting involved in any matters concerning Boeing.

In its complaint, CREW cited earlier reports by Politico and Bloomberg claiming Shanahan lobbied the inclusion of Boeing fourth-generation F-15X fighter aircraft in the newly-released Pentagon budget request. The budget proposal released Tuesday envisions Pentagon buying eight F-15X jets and up to 80 aircraft over the next five years. The proposal, according to Bloomberg, was championed by Shanahan and eventually came through despite reportedly facing strong opposition from the Air Force which reportedly called the jet "ineffective against enemies like China and Russia." The initial budget proposal did not include the funding for the jets, according to Air Force Secretary Heather Wilson, who subsequently resigned.

Reports have also suggested that while promoting the Boeing equipment, Shanahan unloaded heavily on its competitor, Lockheed Martin, and, in particular, on its fifth-generation stealth fighter F-35, beset with delays, groundings and cost overruns has racked up a price tag of $1,5 trillion.

Shanahan reportedly referred to the program as "f***ed up" suggesting that if the contract would go Boeing's way it would have performed "much better".

"Acting Secretary Shanahan's conduct regarding Boeing and its competitors indicate that he may have violated his Ethics Pledge for the same reasons that they raise significant concerns of an appearance of a lack of impartiality," CREW wrote in the complaint to the Department of Defense Acting Inspector General Glenn Fine, asking him to investigate a possible violation of ethics rules by Shahanan. During his three decades at Boeing, Shanahan, among his many occupations, worked as senior vice president of Commercial Airplane Programs. Among his duties on that job was "managing profit and loss for the 737, 747, 767, 777 and 787 programs," his Pentagon profile states.

Shanahan has denied allegations of bias towards his former employer in a statement on Wednesday, saying that he has continued to stick to his ethics agreement "at all times."

A potential new scandal comes at a time when Boeing is struggling to contain a fallout from the recent crash in Ethiopia, which killed 157 people and triggered a wave of temporary bans of the troubled 737 MAX 8 jet by all major nations and airlines. The controversy fueled by safety concerns saw the company's stock plummeting for the third day in a row after the US joined other nations in grounding the planes.